In geothermal power generation, high-temperature geothermal water is collected from a production well, and power is generated using steam separated from the geothermal water. The geothermal water from which steam has been separated is returned into the ground from an reinjection well.
Geothermal water collected from a production well contains more calcium ions and dissolved silica than well water or river water. Therefore, deposition of scale comprising calcium carbonate or amorphous silica readily occurs. In particular, in an above-ground section or in an reinjection well, inhibition of silica scale generated by a decrease in temperature of geothermal water at the above-ground section is a problem to be addressed.
Sulfuric acid injection is generally used to inhibit silica scale. Sulfuric acid injection is a method in which the pH of geothermal water is lowered, whereby the speed of polymerization of silica is reduced and the speed of silica scale deposition is decreased.
However, lowering the pH of geothermal water merely lowers the speed of polymerization of silica. Therefore, in instances in which time is required to return the geothermal water to the reinjection well, deposition of silica scale is not necessarily inhibited to a sufficient extent. In addition, there was a risk of piping or other elements corroding due to the acid. In addition, in instances in which sulfuric acid is used as the acid, there was a risk of deposition of anhydrite or other scale.
Non-patent document 1 discloses injecting a chelating agent into the production well, injecting an alkaline agent in the above-ground section, and inhibiting scale deposition. The solubility of amorphous silica is said to increase with increasing pH value, and to rapidly rise in particular at pH8 or above. Therefore, increasing the pH of geothermal water makes it possible to inhibit silica scale deposition. In contrast to the above method in which the speed of silica polymerization is inhibited, the total amount of silica deposition does not increase with passage of time; therefore, this effect is maintained even in the reinjection well. In addition, additionally using a chelating agent for chelating calcium or magnesium in the production well makes it possible to inhibit deposition of calcium carbonate, anhydrite, and magnesium silicate in the production well.